Electronic field effect devices and methods for their manufacture

ABSTRACT

Electronic field effect devices, and methods of manufacture of these electronic field effect devices are disclosed. In particular, there is disclosed an electronic field effect device which has improved electrical properties due to the formation of a highly mobile two-dimensional charge-carrier gas in a simple structure formed from diamond in combination with polar materials.

FIELD OF THE INVENTION

The present invention relates to electronic field effect devices, and tomethods of manufacture of these electronic field effect devices.

BACKGROUND OF THE INVENTION

The present generation of high frequency (HF) and microwave signals isnow mostly based on Si and GaAs devices. Due to physical limitations,these devices cannot achieve power levels higher than a few hundredwatts (depending on the frequency to be amplified) in simple solid-statedevice configurations. Wide band gap materials (diamond, SiC, GaN, etc),in principle, allow for higher power amplification per unit gate lengthat microwave frequencies. This is because a larger bias voltage, andhence the voltage amplitude on the microwave signal, can be supportedacross the transistor channel region over which the current ismodulated. In effect, the higher breakdown electric field of a wide bandgap semiconductor is exploited. In microwave transistors, the ability tosupport high voltage is particularly desirable since, generally, powerhas to be transferred to a relatively high impedance (50Ω) load.

The use of diamond in manufacturing transistors of various types hasbeen described in, for example, JP-A-60246627, EP 0 343 963 B1 and WO2006/117621 A1.

WO 2006/117621 A1 discloses a metal semiconductor field-effecttransistor (MESFET). The MESFET is manufactured by providing a singlecrystal diamond material substrate having a growth surface on whichfurther layers of diamond material can be deposited, depositing aplurality of further diamond layers on the substrate growth surface, andattaching appropriate contacts to the respective diamond layers, therebydefining a transistor structure. The further diamond layers deposited onthe substrate include a boron doped interface layer (a “delta-doped”layer). Such a design presents several synthesis challenges. The mainchallenge is the requirement to produce nanometer-thin boron layerswhich transition very abruptly to an intrinsic layer (e.g. a change in Bconcentration from about 10¹⁵ B atoms per cm³ to about 10²⁰ B atoms percm³ in a few nm). Growing such boron layers (delta layers) is dependentupon a number of crucial steps including substrate surface preparationand diamond growth conditions. In addition to the synthesis challenges,certain aspects of the device design are not ideal. In particular, theholes (acting as charge carriers) are essentially localised in thevicinity of the acceptors, which leads to an increase in impurityscattering and an overall degradation in the mobility.

U.S. Pat. No. 5,506,422 discloses a diamond-based three-terminaljunction device that uses a material with a wider band gap than diamondto enhance the blocking properties of the gate contact. The disclosurestates that conduction from the source to the drain is confined to aboron-doped layer. The use of the wide band gap material in the gatecontact is not fundamental to the operation of the device, but simply ameans of enhancing its performance by reducing leakage under reversebias. By using the boron-doped layer as the channel the device of U.S.Pat. No. 5,506,422 does not exploit the superior charge carrierproperties of intrinsic diamond compared with boron doped diamond.

Vogg et al (Journal of Applied Physics, vol 96 (2004), 895-902) andNebel et al (Diamond and Related Materials, vol 12 (2003), 1873-1876)disclose pn junction diodes made from {100} and {111} diamond with anepitaxial layer of aluminium nitride. For both orientations of diamondsubstrate surface, it is reported in Vogg et al that there is asignificant lattice parameter mismatch (−13% and +23% for {100} and{111} respectively), and that the AlN layers have a domain structure.Nebel et al reports, “The leakage current in the reverse direction iscaused by imperfections of the pn heterojunction, probably caused bydislocations”. The carrier path of the device described is across theinterface from the p-type doped diamond layer to the n-type doped AlNlayer. This suggests that the structure of the interface between thediamond substrate and the AlN layer is highly defective, something thatwould be extremely detrimental to performance of an electronic devicewhere charge flows across the interface.

Accordingly, it is an object of the invention to provide an alternativedevice structure, and a method of manufacture thereof, having particularadvantages in terms of device manufacture and performance. Anotherobject of the invention is to provide an alternative device structureand a method of manufacture thereof in which the charge carriers and anyionised acceptors/donors are spatially separated.

In III-V systems such as GaAs and GaN, spatial separation of the chargecarriers and ionised acceptors or donors can be achieved by modulationdoping. This is facilitated by the ability to form heterostructuresthrough alloying, i.e. the addition of one or more group III or Velements into the matrix such as In or Al to change the energy band-gapof the material, whilst retaining essentially the same crystal structureacross the interface. In silicon, a group IV element, heterostructurescan be formed by alloying Si with Ge, another group IV element. Alloyingof diamond with Si would form SiC. As SiC has a smaller band gap thandiamond, such a heterostructure formed with diamond would lead to chargecarrier confinement within the SiC layer and the superior electronicproperties of the diamond would not be exploited.

A heterostructure is characterised by a lattice match (that isessentially the same lattice type, essentially the same latticeorientation and with a lattice parameter that is closely matched betweenthe two materials) across the interface of the two distinct materials,and in normal usage typically refers to the situation where there islattice continuity and the interface is defined by a distinct change inthe relative concentrations of alloy components. This does not excludethe possibility of a small difference in the lattice parameter fromwhich could give rise to an array of “misfit dislocations” at theinterface. A related concept is that of heteroepitaxial growth, wherethe lattices on the two sides of the boundary may be distinct, but atthe interface there is a substantial match or registration between thelattices.

The publication ‘Present Status and Future Prospect of WidegapSemiconductor High-Power Devices’, Japanese Journal of Applied Physics,45 (2006), 7565-7586 provides a useful summary of prior art. FIG. 6 ofthe publication discloses a GaN—Al_(x)Ga_((1-x))N heterostructure inwhich a polar Al_(x)Ga_((1-x))N layer is heteroepitaxially grown on apolar GaN layer to produce a potential well for confining electrons.

SUMMARY OF THE INVENTION

The present invention provides an electronic field effect devicecomprising:

-   -   an interface between two materials, wherein the interface is        formed by:        -   a first diamond layer, at least a first surface of which            comprises crystalline intrinsic diamond, and        -   a second layer disposed on the first surface of the first            layer, wherein the second layer is polar; wherein    -   there is a discontinuity in polarisation between the first layer        and the second layer; and    -   there is a band offset between the first layer and the second        layer, in the band in which the dominant charge carriers are        present, such that the dominant charge carriers are confined to        a planar region within the first layer and in close proximity to        the interface by the combined effects of a polarization induced        sheet charge and an electric field provided by the discontinuity        in polarization between the first layer and the second layer,        and the band offset.

The dominant charge carriers provide the useful characteristics of thedevice.

The planar region comprises a region having two dimensions which arevery much larger than the third.

The charge carriers in the device under operation are confined to theplanar region in the first diamond layer and move predominantly in adirection parallel to the interface between the first layer and thesecond layer. Therefore any defects such as dislocations that arepresent in the second layer adjacent to the interface have little effecton the performance of the device.

The electric field provided by the discontinuity in polarization betweenthe second layer and the first layer attracts the carriers towards thepolar material and thus the interface.

The dominant carriers may be present in either the valence band or theconduction band.

The band offset inhibits the carriers from entering the polar material.

At least one of the first layer and the second layer may include a dopedregion. The doped region then provides carriers which can move to andsubsequently be confined to the planar region.

In a preferred embodiment, instead of doping either of the first andsecond layers, the electric field provided by the discontinuity inpolarization between the second layer and the first layer may cause thevalence band maximum to intersect the Fermi level. In such a case, holesare generated at the interface which act as carriers which can move toand subsequently be confined to the planar region. In such a case, the2-dimensional charge carrier gas is formed from thermally generatedcarriers and no dopant is required in either the first layer or thesecond layer.

Alternatively, instead of doping either of the first and second layers,the electric field provided by the discontinuity in polarization betweenthe second layer and the first layer may cause the conduction bandminimum to intersect the Fermi level. In such a case, electrons aregenerated at the interface which act as carriers which can move to andsubsequently be confined to the planar region. In such a case, the2-dimensional charge carrier gas is formed form thermally generatedcarriers and no dopant is required in either the first layer or thesecond layer.

When a dopant is present in the first layer, it preferably resides in alayer which is further away from the interface than the planar region,and sufficiently remote from the planar region to avoid the dopantsignificantly impeding the mobility of the carriers and thus degradingthe device performance. Thus the doped region in the first layer ispreferably at least 2 nm from the interface, more preferably at least 3nm from the interface, more preferably at least 5 nm from the interface,more preferably at least 10 nm from the interface, more preferably atleast 20 nm from the interface, more preferably at least 50 nm from theinterface, and more preferably at least 100 nm from the interface. Thedistance between the interface and the doped region may be influenced bythe dopant concentration in the doped layer to ensure the correct numberdensity of charge carriers in the 2-dimensional charge carrier gas,where the meaning of the term “2-dimensional charge carrier gas” is asis normally understood in the art.

In a preferred embodiment, the first layer comprises an intrinsicdiamond layer including a doped region positioned a distance from theinterface such that the polar layer and the doped region are separatedby an intrinsic diamond channel layer. The doped region in the intrinsicdiamond may comprise an n-type region or a p-type region. When the dopedregion comprises a p-type region, the charge carriers are holes. Thep-type doped region may comprise a boron-doped region. When the dopedregion comprises an n-type region, the charge carriers are electrons.The n-type doped region may comprise a nitrogen-doped region or aphosphorus-doped region.

Unlike the MESFET disclosed in WO 2006/117621 A1, the doped layer doesnot have to be a delta-layer and does not have to be atomically abruptwith respect to the intrinsic diamond channel. Preferably, the squaremodulus of the wave function of the charge carriers (|φ(z)|²) does notoverlap with the doped layer, but resides mostly within the intrinsiclayer. The thicknesses of each of the layers of the device may belimited by the requirement that the doped layer must be capable of beingfully depleted by the gate so that the transistor can be operated inpinch-off mode.

The doped region is preferably prepared by the technique of in-situdoping (that is doping during the crystal growth process). Inparticular, the doped region is preferably prepared in the first diamondlayer by in-situ doping, preferably using boron atoms as the dopantspecies.

Alternatively, the doped region may be prepared by the technique of ionimplantation. In particular, the doped region may be prepared in thefirst diamond layer by ion implantation, preferably of boron ions oratoms.

At the interface, the two layers may be in direct contact such thatthere is no other material interposed. Alternatively, a thin interfacematerial may be provided between the two layers. Preferably such aninterface material forms a layer which is less than 5 nm thick, morepreferably less than 2 nm thick, more preferably less than 1 nm thick.The interface material may be between 0.1 nm and 5 nm thick. Theinterface material may be a different material to both the first andsecond layers. The interface material may be amorphous or crystalline innature. Preferably the interface material is a non-rigid, compliantmaterial. The interface material may be atomic hydrogen. Such a layermay be formed by hydrogenation. Preferably the interface material is aninsulating material. An example of an interface layer might be 0.5 nm ofamorphous silicon dioxide (SiO₂). The interface material may be providedin order to passivate or accommodate dangling bonds at the interface.Such dangling bonds may trap charge carriers, increase charge carrierscattering, and/or reduce mobility and conductivity.

The first diamond layer is a wide band-gap semiconducting material,preferably non-polar, more preferably single crystal diamond, morepreferably single crystal CVD diamond, and more preferably singlecrystal CVD diamond with material properties conforming to thedisclosures in WO 01/96633 and WO 01/96634.

The second layer preferably has a wider bandgap than the first layer,and more preferably comprises MN in its wurtzite phase. An alternativesecond layer may comprise Al_(x)Ga_((1-x))N where x≧0.8.

Since there are two types of charge carrier possible in such a device,electrons and holes, then the device can take two distinct forms.

In the case where the dominant charge carrier type is holes, the holesare confined to the planar region by the combined effects of apolarization field provided by the polar layer, and a valence bandoffset between the first and second layers, wherein the polarizationfield has a polarization vector which points away from the interface andwhich leads to the attraction of the holes towards the interface, andthe valence band offset forms a potential energy barrier inhibitingholes from entering the polar layer, wherein the valence band maximum ofthe second layer is less than the valence band maximum of the firstlayer.

In the case where the dominant charge carrier type is electrons, theelectrons are confined to the planar region by the combined effects of apolarization field provided by the polar layer, and a conduction bandoffset between the first and second layers, wherein the polarizationfield has a polarization vector which points towards the interface andwhich leads to the attraction of the electrons towards the interface,and the conduction band offset forms a potential energy barrierinhibiting electrons from entering the polar layer, wherein theconduction band minimum of the second layer is greater than theconduction band minimum of the first layer.

In a preferred embodiment of the invention, the band gap of the secondlayer is larger than that of the first layer.

In a further preferred embodiment, the band gap of the second layer islarger than that of the first layer, and the band offset in theconduction band and valence band is such that both electrons and holesare inhibited from entering the polar material, that is the valence bandmaximum of the second layer is less than the valence band maximum of thefirst layer and the conduction band minimum of the second layer isgreater than the conduction band minimum of the first layer. Thissituation is known as a “type I band offset”.

When the dominant carriers are holes, the valence band offset movingfrom the first layer to the second layer is negative and its modulus ispreferably greater than 0.05 eV, more preferably greater than 0.1 eV,more preferably greater than 0.15 eV, more preferably greater than 0.2eV, more preferably greater than 0.3 eV, more preferably greater than0.4 eV, and more preferably greater than 0.5 eV.

In principle, there is no upper limit to the modulus (or largestnegative value) of the applicable valence band offset, provided that thedevice is suitably designed such that the holes are retained in anintrinsic region of the first layer. There is a practical limit in thecurrent availability of materials, of about 2 eV, but this is notlimiting on the scope of the present invention.

When the dominant carriers are electrons, the conduction band offsetmoving from the first layer to the second layer is positive andpreferably greater than +0.05 eV, more preferably greater than +0.1 eV,more preferably greater than +0.15 eV, more preferably greater than +0.2eV, more preferably greater than +0.3 eV, more preferably greater than+0.4 eV, and more preferably greater than +0.5 eV.

In principle, there is no upper limit to the modulus (or largestpositive value) of the applicable conduction band offset, provided thatthe device is suitably designed such that the electrons are retained inan intrinsic region of the first layer. There is a practical limit inthe current availability of materials, of about 2 eV, but this is notlimiting on the scope of the present invention.

When the bandgap of the second layer is larger than the bandgap of thefirst layer, then it is preferably larger by 0.1 eV, more preferablylarger by 0.2 eV, more preferably larger by 0.3 eV, more preferablylarger by 0.4 eV, more preferably larger by 0.6 eV, more preferablylarger by 0.8 eV, more preferably larger by 1.0 eV, and more preferablylarger by 1.2 eV. For the purposes of the present invention, the bandgapof diamond is taken to be 5.47 eV.

In principle, there is no upper limit to the bandgap of the secondlayer, provided that the device is suitably designed such that thecarriers are retained in an intrinsic region of the first layer. Thereis a practical limit in the current availability of materials, of about9 eV, but this is not limiting on the scope of the present invention.

The device may comprise a three-terminal device. Preferably, the devicecomprises a field-effect transistor (FET). In view of the fact that inthis device the polar layer leads to the spatial separation andconfinement of the charge carriers in the planar region which leads toan enhancement in the performance of the device, the device shall betermed herein a “Polarization Enhanced-FET” or PE-FET. The PE-FET maycomprise appropriate electric contacts in order to define the transistorstructure. The electrical contacts may comprise a gate, a source and adrain.

Discontinuity in polarisation between the non-polar intrinsic diamondlayer and the polar second layer creates a polarisation-induced sheetcharge a σ_(P)=−(P_(n)) at the interface between the two layers whereP_(n) is the component of polarisation in the polar layer normal to theinterface between the two layers. P_(n) is defined as positive when itpoints away from the interface between the two layers. Such a sheetcharge leads to a significant strength electric field in both thediamond layer and the second polar layer. The sheet charge is not mobilebecause it is a manifestation of the atomic dipoles in the polar secondlayer, which are fixed by the polar material's crystal structure. Theband offset acts as a potential barrier. This potential barrier, coupledwith the electric field results in a triangular potential well which canbe used to attract carriers, resulting in a highly confinedtwo-dimensional electron gas or hole gas in the region of the diamondlayer close to the interface.

The extent of the confined two-dimensional charge carrier gas definesthe region referred to as the planar region. This region is notnecessarily flat over large dimensions, e.g. over dimensions larger than1 μm, more preferably larger than 10 μm, more preferably larger than 100μm, more preferably larger than 1 mm, but will conform to any largescale curvature of the interface between the first and second layer.Sharp features in the interface between the first and second layer,which would result in relatively sharp features in the planar region,are not desirable and may degrade the performance of the device bycausing scattering of the charge carriers. This is controlled by carefulpreparation of the surfaces forming the interface between the first andsecond layer. In particular, the first surface of the first layerpreferably has root-mean-square roughness R_(q) of less than 10 nm,preferably an R_(q) of less than 5 nm, preferably an R_(q) of less than3 nm, preferably an R_(q) of less than 2 nm, preferably an R_(q) of lessthan 1 nm, preferably an R_(q) of less than 0.5 nm preferably an R_(q)of less than 0.3 nm, preferably an R_(q) of less than 0.2 nm, preferablyan R_(q) of less than 0.1 nm. Furthermore, the surface of the secondlayer facing the first layer preferably has an R_(q) of less than 10 nm,preferably an R_(q) of less than 5 nm, preferably an R_(q) of less than3 nm, preferably an R_(q) of less than 2 nm, preferably an R_(q) of lessthan 1 nm, preferably an R_(q) of less than 0.5 nm, preferably an R_(q)of less than 0.3 nm, preferably an R_(q) of less than 0.2 nm, preferablyan R_(q) of less than 0.1 nm.

It is desirable that the first layer has a low dislocation density inthe region of the first surface. In particular, it is desirable that thedensity of dislocations breaking the first surface of the first layer isless than 400 cm⁻², preferably less than 300 cm⁻², preferably less than200 cm⁻², preferably less than 100 cm⁻², measured over an area ofgreater than 0.014 cm², preferably greater than 0.1 cm², preferablygreater than 0.25 cm², preferably greater than 0.5 cm², preferablygreater than 1 cm², and preferably greater than 2 cm².

Methods of preparing and characterising diamond and diamond surfaceswith low dislocation density are reported in the prior art of WO01/96633, WO 01/96634, WO 2004/027123, and co-pending applicationPCT/IB2006/003531. The preferred methods of characterising thedislocation density are the use of a ‘revealing plasma etch’ and the useof x-ray topography.

It is further desirable that the surface of the first layer issubstantially free from damage introduced by post-growth mechanicalprocessing of the as-grown surface to a depth of at least 1 nm,preferably at least 2 nm, preferably at least 5 nm, preferably at least10 nm, preferably at least 20 nm, preferably at least 50 nm, preferablyat least 100 nm, preferably at least 200 nm, preferably at least 500 nm.The presence of such damage, which includes microfractures andmechanically-generated point and extended defects, can have adetrimental effect on the performance of a device through carrierscattering and trapping, perturbation of the local electric field anddegradation of the breakdown electric field.

In the case of diamond and in particular single crystal CVD diamond,such defects can be introduced into the material by mechanicalprocessing of the as-grown surface, such as by using conventionallapping and polishing techniques. These issues are particularly relevantto diamond in view of its hard and brittle nature, and its chemicalresistivity which limits the number of chemical and physical etchingprocesses available. The requirements for processing an electronicsurface for low roughness, and those for processing an electronicsurface for low surface damage are quite distinct. The preparation of anelectronic surface showing both these features is a further aspect ofthis invention.

Generally, thick layers of single crystal CVD diamond in the as-grownstate are not suitable for use as the first layer and their surfaces arenot suitable for use as the first surface because of the presence ofnon-planar features that can develop during growth. Conversely, thediamond layer on which the electronic surface is to be prepared needs tobe sufficiently rigid and robust for processing and handling, andconsequently the fabrication of an electronic device usually starts froma thick diamond layer. There are a number of methods of producing asuitable diamond surface from the as-grown surface of a thick diamondlayer, which processing steps are included in the method. In the contextof this invention, a single crystal CVD layer is considered to be thickwhen its thickness exceeds 20 μm.

Firstly, a surface may be prepared on the thick diamond layer usingmechanical lapping and polishing processes, which have been optimisedfor minimum surface damage by using feedback from, for example, arevealing etch. Such a technique is described in for example WO 01/96633and WO 01/96634. Whilst such a surface may have a low damage level, itis unlikely to be sufficiently free of damage to obtain more thanadequate performance from the device.

The first surface may be prepared from a processed surface, preferablyfrom a mechanically processed surface, preferably a mechanicallyprepared surface itself optimised for minimum surface damage by usingthe method above, by using a further processing stage comprisingchemical etch or other forms of etching, such as ion beam milling,plasma etching or laser ablation, and more preferably plasma etching.Preferably the etching stage removes at least 10 nm, preferably at least100 nm, more preferably at least 1 μm, more preferably at least 2 μm,more preferably at least 5 μm, more preferably at least 10 μm.Preferably the etching stage removes less than 100 μm, preferably lessthan 50 μm, preferably less than 20 μm. This further processed surfacepreferably has an R_(q) of less than 10 nm, preferably an R_(q) of lessthan 5 nm, preferably an R_(q) of less than 3 nm, preferably an R_(q) ofless than 2 nm, preferably an R_(q) of less than 1 nm, preferably anR_(q) of less than 0.5 nm preferably an R_(q) of less than 0.3 nm,preferably an R_(q) of less than 0.2 nm, preferably an R_(q) of lessthan 0.1 nm.

Alternatively, the first surface may be prepared from a processedsurface, preferably from a mechanically processed surface, preferably amechanically prepared surface itself optimised for minimum surfacedamage by using the method above, or from an etched surface such asthose described above, by growing a further thin layer of diamond on thesurface, preferably using a CVD process. Prior to deposition of thefurther thin layer of diamond, the processed surface has an R_(q) ofless than 10 nm, preferably an R_(q) of less than 5 nm, preferably anR_(q) of less than 3 nm, preferably an R_(q) of less than 2 nm,preferably an R_(q) of less than 1 nm, preferably an R_(q) of less than0.5 nm preferably an R_(q) of less than 0.3 nm, preferably an R_(q) ofless than 0.2 nm, preferably an R_(q) of less than 0.1 nm.

Such a diamond layer is preferably grown by CVD synthesis, and is thinto limit the formation of macroscopic growth steps. The thickness ofthis layer, grown onto a previously prepared surface, is less than 20μm, preferably less than 10 μm, preferably less than 1 μm, preferablyless than 100 nm, preferably less than 50 nm, preferably less than 20nm, preferably less than 10 nm. The surface of this thin layer is thenused as the first surface in its as grown state.

Such a thin layer may be prepared using a number of techniques includingmonolayer growth techniques and use of off-axis surfaces to control thepropagation of surface steps.

The surface preferably has Miller indices close to those of a {001}surface, as this is a surface upon which homoepitaxial CVD diamondgrowth is most easily accomplished. Alternatively the surface may haveMiller indices close to those of a { 111} surface, as such a surface hasbeen found to be the most suitable for the deposition of AlN layers. Thenormal to the surface is preferably between 0° and about 5°, preferablybetween about 0.5° and about 1°, of the normal to a {001} or a {111}surface. Where the surface is close to a {001} surface, the normal tothe surface is preferably within about 10° of the great circle passingthrough the pole of the {001} surface and the pole of an adjacent {101}surface.

Such a layer preferably comprises high purity intrinsic diamond, morepreferably high purity intrinsic diamond with material propertiesconforming to the disclosures in WO 01/96633 and WO 01/96634

The surface of this thin as-grown layer forms the first surface andpreferably has an R_(q) of less than 10 nm, preferably an R_(q) of lessthan 5 nm, preferably an R_(q) of less than 3 nm, preferably an R_(q) ofless than 2 nm, preferably an R_(q) of less than 1 nm, preferably anR_(q) of less than 0.5 nm preferably an R_(q) of less than 0.3 nm,preferably an R_(q) of less than 0.2 nm, preferably an R_(q) of lessthan 0.1 nm. Thus, this surface has very low surface roughness and inaddition is free of processing damage.

In the case of diamond, the prepared surface onto which this layer maybe grown could be any form of diamond, but is preferably CVD syntheticdiamond, and where the first layer contains a doped layer, morepreferably CVD synthetic diamond with the doped layer already present,more preferably this doped layer already formed by CVD synthesis.

The doped layer may form the surface which is processed prior to thefinal synthesis of the intrinsic semiconductor layer.

The polar layer may comprise a pyroelectric material or a piezoelectricmaterial. However, the primary source of the polar field is from thepyroelectric properties of the polar layer, even if a piezoelectriceffect is also present. The polar layer may comprise AlN which has aband gap energy, E_(g)=6.2 eV, and which would provide an electric fieldstrength of significantly greater than 1 MVcm⁻¹ at the interface betweenthe diamond and AlN layers. AlN offers the advantages of having a wideband gap, being commercially readily available, and having a largespontaneous polarisation and good thermal properties as well asrobustness.

The polar second layer may comprise a composite layer formed of two ormore polar layers. Forming such a composite polar second layer providesan alternative method for controlling the density of the two-dimensionalcarrier gas more accurately. The polar layers are preferably III-V polarmaterials such as AlN or AlGaN. For example, the second layer maycomprise a composite layer comprising an AlN layer with an AlGaN layerformed thereon.

For a material to be a polar layer in its unstressed state, thecrystallographic point group should be one of the following: 1, 2, m,mm2, 3, 3 m, 4, 4 mm, 6 or 6 mm, where the notation is theHermann-Mauguin notation. The preferred material AlN has point group 6mm.

Alternatively, a material having one of the piezoelectriccrystallographic point groups that is not polar could be made polar bydepositing it in a stressed state. The relevant piezoelectriccrystallographic point groups are, in the Hermann-Mauguin notation: 222,4, 422, 42 m, 32, 6, 622, and 62 m.

The direction of the polarization vector P of a polar material is aproperty of its crystal structure. Using aluminium nitride as anexample, the [0001] crystallographic direction is parallel to one set ofAl—N bonds and is defined as being from the Al atom to the N atom. TheAl atom is charged slightly positive and the nitrogen is chargedslightly negative, and, due to spatial arrangement of the atoms aspontaneous polarisation P is formed. In AlN P is always in the [000 1 ]direction.

The identification of candidate materials for the polar layer that meetthe necessary criteria on the band offsets and band gap is not trivial,particularly when the first layer is already such a wide band gapmaterial as diamond. Possible candidate materials include:

-   -   SiO₂ in the form of quartz (point group 32) in the stressed        state, and hexagonal boron nitride (point group 6 mm).

The present invention also provides a method of forming an electronicfield-effect device comprising:

-   -   forming an interface between a first layer and a second layer        by:        -   providing a first diamond layer, at least a first surface of            which comprises crystalline intrinsic diamond; and        -   providing a second layer, wherein the second layer is polar,            and the second layer is arranged on the first surface of the            first layer, wherein:    -   there is a discontinuity in polarisation between the first layer        and the second layer; and    -   there is a band offset between the first layer and the second        layer, in the band in which the dominant charge carriers are        present, such that the dominant charge carriers are confined to        a planar region within the first layer and in close proximity to        the interface by the combined effects of a polarization induced        sheet charge and an electric field provided by the discontinuity        in polarisation between the first layer and the second layer,        and the band offset.

Although conceptually the device comprises a first and second layer,this does not restrict the order of fabrication. For example, the secondlayer may be formed on a pre-existing first layer, for example by PVD orCVD processes, or the first layer may be formed on a pre-existing secondlayer, for example by PVD or CVD processes, or the two layers may beformed completely separately and then bonded together.

The conventional methods of charge carrier confinement used in othersystems, such as heterostructures and modulation doping, are impracticalin diamond. Thus, despite substantial interest in developing deviceswhich exploit the extreme electronic properties of diamond, all efforthas focused on controlled structures within the diamond, such as dopeddelta layers, because alternative options taught by the prior art inother semiconductor systems are not applicable to diamond. However, themethod of this invention provides for forming a highly mobile 2D gas ofcarriers in diamond, using simple diamond structures in combination withother, polar materials.

A particular issue with diamond is that of surface termination. It iswell known that the electron affinity of diamond can be modified withrespect to that of a clean diamond surface by terminating the diamondsurface with hydrogen or oxygen for instance. Since the position of theband offsets between two dissimilar materials can be a function of theelectron affinities of those materials (known in the art as “Anderson'sRule”), the use of a suitably terminated diamond surface could be usedto control the band offsets in such a way as to improve deviceperformance by controlling such parameters as the 2D gas density and thespatial profile of the carrier wave functions. For example, the electronaffinity is believed to be decreased for a hydrogen terminated surfacecompared with an unterminated surface. Thus, whilst an ideal structuremay go directly from a first diamond layer to a second polar layer, forexample AlN, in reality this interface may contain a fractionalmonolayer, a monolayer or multiple monolayers of other chemical speciessuch as H, O, F, OH (including the various isotopic alternatives of eachof these) etc. It may be desirable to deliberately terminate the diamondlayer with a non-carbon species, or in other ways insert such species atthe interface, in order to control the conduction and valence bandoffsets between the diamond layer and the polar layer. This technique isparticularly relevant since there is no requirement for latticecontinuity across the interface. As previously mentioned, an amorphousor atomic hydrogen layer may be provided at the interface layer topassivate any dangling bonds. Such an atomic hydrogen layer may beprovided by hydrogenation.

Where a surface has a specific surface termination, the surface may beterminated such that the proportion of bonds on the surface with thespecific surface termination is preferably about 60% or more, preferablyabout 70% or more, preferably about 80% or more, preferably about 90% ormore.

The first layer may comprise natural diamond or synthetic diamond.Synthetic diamond used in the present invention may be formed by ahigh-pressure-high-temperature (HPHT) process or more preferably bychemical vapour deposition (CVD). If the CVD diamond layer is singlecrystal, then preferably the polar layer, such as AlN is bonded to theCVD diamond layer, or more preferably formed on the CVD diamond layer.Where the CVD diamond layer is polycrystalline, it would be preferable,in some instances, to form the polycrystalline CVD diamond layer ontothe polar layer.

The polar layer may be formed on the diamond layer by deposition, forexample growth by CVD (e.g. MOCVD or MOVPE) or molecular beam epitaxy(MBE). Alternatively, the polar layer may be attached to the diamondlayer by bonding. Since the polar layer must not only be capable ofshowing polarity, but must also provide a polarization-induced electricfield at the interface between the first and second layers, the polarlayer cannot be amorphous or polycrystalline with randomly orientedpolar domains. Preferably, the polar layer comprises polar domains ofgood crystalline quality which are substantially aligned in the desireddirection to provide at the interface a relatively uniformpolarization-induced electric field of the required magnitude andorientation. The most preferred form in this respect would be a singlecrystal polar material. However, well-oriented polycrystalline materialis suitable. Significant variation in the polarization-induced electricfield provided at the diamond surface, and in particular variationscaused by local inversion domains in the polar layer, would provide ascattering mechanism for the carriers and degrade the performance of thedevice. For the example of AlN, either the [0001] or [000 1] direction(depending upon whether the device uses holes or electrons as the chargecarrier) of AlN is preferably normal to the intrinsic diamond surface inorder to align and maximize σ_(P), but a combination of significantdomains of both orientations is highly undesirable.

An advantage of bonding the polar layer to the intrinsic diamond layeris that good quality material of single polarity is readily available.The polar layer may have good quality crystal structure. However, thereis no need to lattice match the polar layer to the intrinsic diamondlayer. The means of bonding must not impact substantially on the deviceperformance. However, the bonding medium may form the additionalinterface layer separating the first and second layers as describedearlier. A technique that may be used to bond the polar layer to anintrinsic diamond layer is anodic bonding.

In a preferred embodiment, the intrinsic diamond layer includes a dopedregion. The intrinsic diamond layer may include a doped layer positioneda distance from the interface such that the polar layer and the dopedlayer are separated by an intrinsic diamond channel layer and the dopedlayer is arranged between the intrinsic diamond channel layer and anintrinsic diamond substrate layer.

The doped layer in the intrinsic diamond may comprise an n-type layer ora p-type layer. When the doped layer comprises a p-type layer, thecharge carriers are holes. The p-type doped layer may comprise a B-dopedlayer. When the doped layer comprises an n-type layer, the chargecarriers are electrons. The n-type doped layer may comprise anitrogen-doped layer or a phosphorus-doped layer. The doped layer may beprepared by implantation, for example implanting B ions, or preferablyit is prepared by CVD synthesis, for example adding gases such asdiborane to the synthesis process. CVD synthesis of the doped layer maytake place in situ with the other layers, or it may take place in aseparate synthesis stage to the intrinsic diamond, particularly theintrinsic diamond layer forming the first surface in which the chargecarriers for the 2 dimensional carrier gas.

When the doped region is a p-type region in the intrinsic diamond layer(i.e. holes are used as carriers), the polar layer is arranged so that acomponent of its polarisation vector P points away from the interfacebetween the intrinsic diamond and the polar layer (i.e. towards thesurface of the device). For example, where the polar layer comprisesAlN, N-polar AlN is used. The polarisation sheet charge formed at theinterface will then be negative and will consequently attract the holesas charge carriers towards the polar layer. The valence band offsetbetween the polar layer and the intrinsic diamond layer may be chosen sothat the holes are prevented from entering the polar layer. The valenceband offset acts as a potential barrier. This potential barrier coupledwith the attractive force leads to an approximately triangular potentialwell in which a two-dimensional hole gas (2DHG) is formed. The hole wavefunction is a maximum in the intrinsic diamond channel layer. Therefore,the 2DHG is localised in the intrinsic diamond channel close to theinterface between the intrinsic diamond channel and the polar layer.Since the 2DHG is formed in the intrinsic diamond channel layer, ratherthan along the interface, the carriers are spatially separated from theionised acceptors and the holes have a very high mobility.

When the doped region is an n-type region in the intrinsic diamondlayer, the polar layer is arranged so that a component of itspolarisation vector P points towards the interface between the intrinsicdiamond and the polar layer (i.e. away from the surface of the device).For example, where the polar layer comprises AlN, Al-polar AlN is used.The polarisation sheet charge formed at the interface will then bepositive and will consequently attract the electrons as charge carrierstowards the polar layer. The conduction band offset between the polarlayer and the intrinsic diamond layer may be chosen so that theelectrons are prevented from entering the polar layer. The conductionband offset acts as a potential barrier. This potential barrier coupledwith the attractive force leads to a triangular potential well in whicha two-dimensional electron gas (2DEG) is formed. The electron wavefunction is a maximum in the intrinsic diamond channel layer. Therefore,the 2DEG is localised in the intrinsic diamond channel close to theinterface between the intrinsic diamond channel and the polar layer.Since the 2DEG is formed in the intrinsic diamond channel layer, ratherthan along the interface, the carriers are spatially separated from theionised donors and the electrons have a very high mobility. Furthermore,performance is further improved because electrons have a higher mobilityin diamond than holes.

In another preferred embodiment, the polar layer includes the dopedregion. The entire polar layer may be doped or part of the polar layermay be doped, this part preferably forming a plane substantiallyparallel to the interface between the first and second layers.Consequently the device may comprise a doped polar layer or anundoped/doped/undoped stack arranged on a first surface of an intrinsicdiamond layer. With this embodiment, the diamond synthesis is lessdemanding because only a high quality intrinsic diamond layer isrequired. This is achievable in volume production. Furthermore, there isno potential problem of incomplete depletion of a doped region withinthe intrinsic diamond layer, which would prevent the transistoroperating in pinch-off mode.

The doped polar layer may comprise an n-type layer or a p-type layer.When the doped polar layer comprises a p-type layer, the charge carriersare holes. The doped polar layer may comprise Al_(x)Ga_(1-x)N doped withMg (where x≧0.8 to ensure E_(g)>5.47 eV). When the doped layer comprisesan n-type layer, the charge carriers are electrons. Electrons as chargecarriers are preferred because they have a higher mobility than holes.The doped polar layer may comprise AlN doped with Si. Alternatively, thedoped polar layer may comprise Al_(x)Ga_(1-x)N doped with Si (wherex≧0.8 to ensure E_(g)>5.47 eV). A further advantage of using AlN orAl_(x)Ga_(1-x)N (where x≧0.8) as the polar layer is that the relativedielectric constant of these materials is very much greater than diamond(>10 compared to 5.7 for diamond). From Gauss' law, the product of therelative dielectric constant (∈_(r)) and the normal electric field oftwo materials has to be constant at their interface. Therefore, duringoperation the peak electric field should be in the intrinsic diamondlayer rather than the AlN (or Al_(x)Ga_(1-x)N) layer. This isadvantageous because the breakdown field strength of diamond issignificantly greater than that of most other materials (10 MVcm⁻¹), sothere will be an associated improvement in the device breakdown voltage(and therefore operating voltage range) enabling a higher peak outputpower compared to alternative device configurations.

When the doped polar layer is an n-type layer, the polar layer isarranged so that a component of its polarisation vector P points towardsthe interface between the intrinsic diamond and the polar layer (awayfrom the surface of the device). The polarisation sheet charge formed atthe interface will then be positive and will consequently result incharge transfer of electrons from the polar layer into the intrinsicdiamond layer. The conduction band offset between the polar layer andthe intrinsic diamond layer acts as a potential barrier. This potentialbarrier coupled with the attractive force leads to an approximatelytriangular potential well in which a two-dimensional electron gas (2DEG)is formed. The electron wave function is a maximum in the intrinsicdiamond layer. Therefore, the 2DEG is localised in the intrinsic diamondlayer close to the interface between the intrinsic diamond layer and thepolar layer. Since the 2DEG is formed in the intrinsic diamond layer,the carriers are spatially separated from the ionised donors and theelectrons have a very high mobility.

When AlN doped with Si is used as the doped polar layer, even though thedonor activation energy in AlN is very large compared to typicaloperating temperatures, the >1 MV cm⁻¹ strength electric field in theAlN layer will ensure field-assisted tunnelling of electrons into thediamond layer. If Al_(x)Ga_(1-x)N doped with Si is used as the dopedpolar layer (with x chosen to ensure the potential well in the intrinsicdiamond layer is maintained), a higher concentration of Si donors couldbe incorporated, and a higher proportion of the donors could be ionised.However, Al_(x)Ga_(1-x)N used as the polar layer instead of AlN wouldlead to a reduced electric field strength and potentially lead to alower band offset, which may in combination lead to a lower 2DEGdensity.

When the doped layer is a p-type layer, the polar layer is arranged sothat a component of its polarisation vector P points away from theinterface between the intrinsic diamond layer and the polar layer(towards the surface of the device). The polarisation sheet chargeformed at the interface will then be negative and will consequentlyresult in charge transfer of holes from the polar layer into theintrinsic diamond layer. The valence band offset between the polar layerand the intrinsic diamond layer acts as a potential barrier. Thispotential barrier coupled with the attractive force leads to atriangular potential well in which a two-dimensional hole gas (2DHG) isformed. The hole wave function is a maximum in the intrinsic diamondlayer. Therefore, the 2DHG is localised in the intrinsic diamond layerclose to the interface between the intrinsic diamond layer and the polarlayer. Since the 2DHG is formed in the intrinsic diamond layer, thecarriers are spatially separated from the ionised acceptors and theholes have a very high mobility.

In addition to doping the first or second layer to provide chargecarriers, the first and/or the second layer may be further doped inorder to provide opposite charges that modify the polarisation inducedelectric field and alter the two-dimensional carrier gas density.

In another preferred embodiment, neither the intrinsic diamond layer northe polar layer includes a doped region.

The polar layer may be arranged so that a component of its polarisationvector P points away from the interface between the intrinsic diamondand the polar layer (i.e. towards the surface of the device). Theelectric field provided by the discontinuity in polarization between thesecond layer and the first layer causes the valence band maximum tointersect the Fermi level. Holes would be generated near the interfacewhich act as carriers. For example, where the polar layer comprises AlN,N-polar AlN is used. The polarisation sheet charge formed at theinterface will then be negative and will consequently attract the holesas charge carriers towards the polar layer. The valence band offsetbetween the polar layer and the intrinsic diamond layer would be chosenso that the holes are prevented from entering the polar layer. Thevalence band offset acts as a potential barrier. This potential barriercoupled with the attractive force leads to an approximately triangularpotential well in which a two-dimensional hole gas (2DHG) is formed. Thehole wave function is a maximum in the intrinsic diamond channel layer.Therefore, the 2DHG is localised in the intrinsic diamond channel closeto the interface between the intrinsic diamond channel and the polarlayer.

Alternatively, the polar layer may be arranged so that a component ofits polarisation vector P points towards the interface between theintrinsic diamond and the polar layer (i.e. away from the surface of thedevice). The electric field provided by the discontinuity inpolarization between the second layer and the first layer causes theconduction band minimum to intersect the Fermi level. Electrons would begenerated near the interface which act as carriers. For example, wherethe polar layer comprises AlN, Al-polar AlN is used. The polarisationsheet charge formed at the interface will then be positive and willconsequently attract the electrons as charge carriers towards the polarlayer. The conduction band offset between the polar layer and theintrinsic diamond layer may be chosen so that the electrons areprevented from entering the polar layer. The conduction band offset actsas a potential barrier. This potential barrier coupled with theattractive force leads to a triangular potential well in which atwo-dimensional electron gas (2DEG) is formed. The electron wavefunction is a maximum in the intrinsic diamond channel layer. Therefore,the 2DEG is localised in the intrinsic diamond channel close to theinterface between the intrinsic diamond channel and the polar layer.Furthermore, performance is further improved because electrons have ahigher mobility in diamond than holes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described withreference to the following drawings in which:

FIG. 1 shows a first embodiment of a FET according to the presentinvention;

FIG. 2 shows an energy band diagram for the interface between the polarand diamond layers shown in FIG. 1;

FIG. 3 shows a second embodiment of a PEFET according to the presentinvention;

FIG. 4 shows an energy band diagram for the interface between the polarand diamond layers shown in FIG. 3;

FIG. 5 shows a third embodiment of a PEFET according to the presentinvention;

FIG. 6 shows an energy level band for the interface between the polarand diamond layers shown in FIG. 5;

FIG. 7 shows a fourth embodiment of a PEFET according to the presentinvention;

FIG. 8 shows an energy band diagram for the interface between the polarand diamond layers shown in FIG. 7;

FIGS. 9A and 9B show a fifth embodiment of a PEFET according to thepresent invention.

FIG. 10 shows an energy band diagram for the interface between the polarand diamond layers shown in FIG. 9.

DETAILED DESCRIPTION OF THE DRAWINGS

A first embodiment of the present invention is shown in FIG. 1. A polarlayer 20 is disposed on an intrinsic diamond layer 10 so that itspolarisation vector points substantially away from the interface betweenthe polar layer 20 and the intrinsic diamond layer 10. The polar layeris either grown or bonded onto the intrinsic diamond layer. The polarlayer 20 has a band gap greater than that of the intrinsic diamond layer10. The intrinsic diamond layer has a band gap, E_(g), of 5.47 eV. Thepolar layer may comprise an N-polar AlN layer with a band gap of 6.2 eV.The intrinsic diamond layer 10 includes a B-doped layer 40 of thicknesst positioned a distance d from the interface between the intrinsicdiamond layer 10 and the polar layer 20. Thickness t and distance d arechosen such that the doped layer can be fully depleted by the gate 90and pinch-off can be achieved. The B-doped layer splits the intrinsicdiamond layer into an intrinsic diamond channel 30 and an intrinsicdiamond substrate 50. A gate contact 90 is arranged on the polar layer20 and a source 92 and a drain 94 are arranged to contact the intrinsicdiamond channel 30.

With an N-polar AlN layer, the discontinuity in polarisation between theintrinsic diamond layer 10 and the polar layer 20 creates a negativepolarisation sheet charge −σ_(P) at the interface between the twolayers. This sheet charge leads to a significant strength electricfield. For an AlN layer as the polar layer, the electric field strengthin the intrinsic diamond channel 30 is estimated to be about 8 MVcm⁻¹.

An energy band diagram for the interface between the polar layer and theintrinsic diamond channel is shown in FIG. 2.

The B-doped layer 40 acts as a source of holes and the electric fieldresulting from the polarisation sheet charge −σ_(P) attracts the holesfrom the B-doped layer 40 towards the interface with the polar layer 20,along path 64 as shown in FIG. 2. The discontinuity 68 in the valenceband maxima between the diamond layer 10 and the polar layer 20 acts asa potential barrier which prevents transfer of holes from the diamondlayer into the polar layer. The holes 62 are trapped in a triangularpotential well in the intrinsic diamond channel close to the interfacewith the polar layer. This results in a highly confined 2DHG in region60 of the intrinsic diamond channel close to the interface.

The square modulus of the hole wave function |φ(z)|² 66 is a maximum inthe diamond layer, rather than at the interface. Therefore, transport ofholes occurs in the high purity and high crystalline quality intrinsicdiamond channel rather than along the doped region or the polar layer oralong the interface itself, any of which would lead to significantcarrier scattering effects.

A second embodiment of the present invention is shown in FIG. 3. A polarlayer 25 is disposed on an intrinsic diamond layer 10 so that itspolarisation vector points substantially towards the interface betweenthe polar layer 25 and the intrinsic diamond layer 10. The polar layeris either grown or bonded onto the intrinsic diamond layer. The polarlayer 25 has a band gap greater than that of the intrinsic diamond layer10. The intrinsic diamond layer has a band gap, E_(g), of 5.47 eV. Thepolar layer may comprise an Al-polar AlN layer with a band gap of 6.2eV. The intrinsic diamond layer 10 includes an n-type doped layer 45,such as phosphorus or nitrogen, of thickness t positioned a distance dfrom the interface between the intrinsic diamond layer 10 and the polarlayer 20. Thickness t and distance d are chosen so that the doped layercan be fully depleted by the gate 90 and pinch-off can be achieved. Then-type doped layer 45 splits the intrinsic diamond layer 10 into anintrinsic diamond channel 30 and an intrinsic diamond substrate 50. Agate contact 90 is arranged on the polar layer 25 and a source 92 and adrain 94 are arranged to contact the intrinsic diamond channel 30.

With an Al-polar AlN layer, the discontinuity in polarisation betweenthe intrinsic diamond layer 10 and the polar layer 25 creates a positivepolarisation sheet charge +σ_(P) at the interface between the twolayers. This sheet charge leads to a significant strength electricfield. For an AlN layer as the polar layer, the electric field strengthin the intrinsic diamond channel 30 is estimated to be about 8 MVcm⁻¹.

An energy band diagram for the interface between the polar layer and theintrinsic diamond channel is shown in FIG. 4.

The n-type doped layer 45 acts as a source of electrons and the electricfield resulting from the polarisation sheet charge +σ_(P) attracts theelectrons from the n-type doped layer 45 towards the interface with thepolar layer 25, along path 74 as shown in FIG. 4. The discontinuity 78in the conduction band minimum between the diamond layer 10 and thepolar layer 25 acts as a potential barrier which prevents transfer ofelectrons from the diamond layer into the polar layer. The electrons 72are trapped in an approximately triangular potential well in theintrinsic diamond channel close to the interface with the polar layer.This results in a highly confined 2DEG in region 70 of the intrinsicdiamond channel close to the interface.

The square modulus of the electron wave function |φ(z)|² 76 is a maximumin the diamond layer, rather than at the interface. Therefore, transportof electrons occurs in the high purity and high crystalline qualityintrinsic diamond channel rather than along the doped region or thepolar layer or along the interface itself, either of which would lead tosignificant carrier scattering effects.

A third embodiment of the present invention is shown in FIG. 5. Ann-type doped polar layer 120 is disposed on an intrinsic diamond layer110 so that its polarisation vector points substantially towards theinterface between the polar layer 120 and the intrinsic diamond layer110. The n-type doped layer does not have to be doped uniformly; it mayjust contain a doped layer within it. The n-type doped polar layer iseither epitaxially grown or bonded onto the intrinsic diamond layer. Then-type doped polar layer 120 has a band gap greater than that of theintrinsic diamond layer 110. The intrinsic diamond layer has a band gap,E_(g), of 5.47 eV. The n-type doped polar layer may comprise a Si dopedAl-polar AlN layer. The thickness of the doped polar layer is believedto be limited by the need for the transistor to function in pinch-offmode. A gate contact 190 is arranged on the polar layer 120 and a source192 and a drain 194 are arranged to contact the intrinsic diamond layer110.

With an Al-polar Si doped AlN layer, the discontinuity in polarisationbetween the intrinsic diamond layer 110 and the doped polar layer 120creates a positive polarisation-induced sheet charge +σ_(P) at theinterface between the two layers. This sheet charge leads to asignificant strength electric field.

An energy band diagram for the interface between the doped polar layerand the intrinsic diamond channel is shown in FIG. 6.

The n-type doped polar layer 120 acts as a source of electrons and theelectric field resulting from the polarisation sheet charge +σ_(P)results in the transfer, which may be by tunnelling, of electrons fromthe doped polar layer 120 through the interface and into the intrinsicdiamond layer 110, along path 164 as shown in FIG. 6. The discontinuity168 in the conduction band minima between the diamond layer 110 and thepolar layer 120 acts as a potential barrier which contains the electronsin the intrinsic diamond layer. The electrons 162 are trapped in atriangular potential well in the intrinsic diamond channel close to theinterface with the polar layer. This results in a highly confined 2DEGin region 160 of the intrinsic diamond layer 110 close to the interface.

The square modulus of the electron wave function |σ(z)| 166 is a maximumin the diamond layer, rather than at the interface. Therefore, transportof electrons occurs in the high purity and high crystalline qualityintrinsic diamond channel rather than along the doped region or thepolar layer or along the interface itself, either of which would lead tosignificant carrier scattering effects.

A fourth embodiment of the present invention is shown in FIG. 7. Ap-type doped polar layer 125 is disposed on an intrinsic diamond layer110 so that its polarisation vector points substantially away from theinterface between the polar layer 125 and the intrinsic diamond layer110. The p-type doped layer does not have to be doped uniformly; it mayjust contain a doped layer within it. The p-type doped polar layer iseither epitaxially grown or bonded onto the intrinsic diamond layer. Thep-type doped polar layer 125 has a band gap greater than that of theintrinsic diamond layer 110. The intrinsic diamond layer has a band gap,E_(g), of 5.47 eV. The p-type doped polar layer may comprise an Mg-dopedN-polar AlN layer. The thickness of the doped polar layer is believed tobe limited by the need for the transistor to function in pinch-off mode.A gate contact 190 is arranged on the polar layer 125 and a source 192and a drain 194 are arranged to contact the intrinsic diamond layer 110.

With a p-type doped N-polar AlN layer, the discontinuity in polarisationbetween the intrinsic diamond layer 110 and the doped polar layer 125creates a negative polarisation-induced sheet charge −σ_(P) at theinterface between the two layers. This sheet charge leads to asignificant strength electric field.

An energy band diagram for the interface between the doped polar layerand the intrinsic diamond channel is shown in FIG. 8.

The p-type doped polar layer 125 acts as a source of holes and theelectric field resulting from the polarisation sheet charge −σ_(P)results in the transfer of holes from the doped polar layer 125 throughthe interface and into the intrinsic diamond layer 110, along path 174as shown in FIG. 8. The discontinuity 178 in the valence band maximabetween the diamond layer 110 and the polar layer 125 acts as apotential barrier which contains the holes 172 in the intrinsic diamondlayer. The holes 172 are trapped in a triangular potential well in theintrinsic diamond channel close to the interface with the polar layer.This results in a highly confined 2DHG in region 170 of the intrinsicdiamond layer 110 close to the interface.

The square modulus of the hole wave function |φ(z)| 176 is a maximum inthe diamond layer, rather than at the interface. Therefore, transport ofholes occurs in the high purity and high crystalline quality intrinsicdiamond channel rather than along the doped region or the polar layer oralong the interface itself, either of which would lead to significantcarrier scattering effects.

A fifth embodiment of the present invention is shown in FIGS. 9A and 9B.A polar layer 225 is disposed on an intrinsic diamond layer 210 so thatits polarisation vector points substantially away from the interfacebetween the polar layer 225 and the intrinsic diamond layer 210. As seenin FIG. 9B, an interface material 300 may optionally be provided betweenthe two layers. The polar layer 225 has a band gap greater than that ofthe intrinsic diamond layer 210. The intrinsic diamond layer has a bandgap, E_(g), of 5.47 eV. The polar layer may comprise an N-polar AlNlayer. The thickness of the polar layer is believed to be limited by theneed for the transistor to function in pinch-off mode. A gate contact290 is arranged on the polar layer 225 and a source 292 and a drain 294are arranged to contact the intrinsic diamond layer 210.

With an N-polar AlN layer, the discontinuity in polarisation between theintrinsic diamond layer 210 and the polar layer 225 creates a negativepolarisation-induced sheet charge −σ_(P) at the interface between thetwo layers. This sheet charge leads to a significant strength electricfield which may causes the valence band maximum to intersect the Fermilevel (E_(F)). Holes would be generated near the interface which act ascarriers.

An energy band diagram for the interface between the polar MN layer andthe intrinsic diamond layer is shown in FIG. 10.

The electric field resulting from the polarisation sheet charge −σ_(P)results in the generation of holes in the diamond layer 210 near theinterface as shown in FIG. 10. The discontinuity 278 in the valence bandmaxima between the diamond layer 210 and the polar layer 225 acts as apotential barrier which contains the holes in the intrinsic diamondlayer. The holes are trapped in a triangular potential well in theintrinsic diamond channel close to the interface with the polar layer.This results in a highly confined 2DHG in region 270 of the intrinsicdiamond layer 210 close to the interface.

The square modulus of the hole wave function |φ(z)| 276 is a maximum inthe diamond layer, rather than at the interface. Therefore, transport ofholes occurs in the high purity and high crystalline quality intrinsicdiamond channel rather than along the polar layer or along the interfaceitself, either of which would lead to significant carrier scatteringeffects.

It will of course be understood that the present invention has beendescribed above purely by way of example, and that modifications ofdetail can be made within the scope of the invention as defined by theclaims.

The invention claimed is:
 1. An electronic field effect devicecomprising: an interface between two materials, wherein the interface isformed by: a first undoped diamond layer, at least a first surface ofwhich comprises crystalline intrinsic diamond, and a second undopedlayer disposed on the first surface of the first layer, wherein thesecond layer is polar; wherein there is a discontinuity in polarizationbetween the first layer and the second layer; and there is a band offsetbetween the first layer and the second layer, in the band in which thedominant charge carriers are present, such that the dominant chargecarriers are confined to a planar region within the first layer and inclose proximity to the interface by the combined effects of apolarization induced sheet charge and an electric field provided by thediscontinuity in polarization between the first layer and the secondlayer, and the band offset.
 2. The electronic field-effect device ofclaim 1 wherein the second layer comprises a pyroelectric layer.
 3. Theelectronic field-effect device of claim 1 wherein the second layercomprises AlN or Al_(x)Ga_(1-x)N.
 4. The electronic field-effect deviceof claim 1 wherein the diamond layer comprises synthetic diamond formedby chemical vapour deposition.
 5. The electronic field-effect device ofclaim 1 wherein the second layer is arranged so that a component of itspolarization vector P points away from the interface.
 6. The electronicfield-effect device of claim 5 wherein the second layer is arranged sothat its polarization vector P is substantially normal to the interface.7. The electronic field-effect device of claim 1, further comprising aninterface material provided between the first layer and the secondlayer.
 8. The field effect device of claim 1 wherein the first surfaceof the first diamond layer is formed by etching.
 9. The field effectdevice of claim 8 wherein the etching is performed by plasma etching.10. The field effect device of claim 1 wherein the first surface of thefirst diamond layer is formed by deposition.
 11. The field effect deviceof claim 10 wherein the first surface of the first diamond layer isprepared from a processed surface with an R_(q) of less than 10 nm bygrowing a further thin layer using a CVD process, wherein the thicknessof this layer, grown onto the previously prepared surface, is less than20 μm.
 12. The field effect device of claim 1 wherein the first surfaceof the first diamond layer has a dislocation density determined usingthe method of X-ray topography of less than 400 cm⁻² over an areagreater than 0.014 cm².
 13. The field effect device of claim 1 whereinthe first surface of the first layer has an Rq less than 1 nm.
 14. Amethod of forming an electronic field-effect device comprising: formingan interface between a first layer and a second layer by: providing afirst undoped diamond layer, at least a first surface of which comprisescrystalline intrinsic diamond; and providing a second undoped layer,wherein the second layer is polar, and the second layer is arranged onthe first surface of the first layer, wherein: there is a discontinuityin polarization between the first layer and the second layer; and thereis a band offset between the first layer and the second layer, in theband in which the dominant charge carriers are present, such that thedominant charge carriers are confined to a planar region within thefirst layer and in close proximity to the interface by the combinedeffects of a polarization induced sheet charge and an electric fieldprovided by the discontinuity in polarization between the first layerand the second layer, and the band offset.
 15. The method of claim 14wherein the step of providing a first layer comprises forming a diamondlayer by chemical vapour deposition (CVD).
 16. The method of claim 14wherein the step of providing the second layer comprises growing ordepositing the second layer on the first surface of the first layer. 17.The method of claim 14 wherein the step of providing the second layercomprises bonding the second layer to the first surface of the firstlayer.